1. Field
The invention relates to systems and methods for improving the performance of 90 degree coplanar waveguide (CPW) bends at mm-wave frequencies. More particularly, the CPW bends may be chamfered on the signal conductor and the ground plane and additional vias may be placed near the CPW bends.
2. Background
Microwave and mm-wave RF circuits may be integrated on a dielectric substrate with transmission lines (e.g., CPW) that feed the RF signals between the circuits. Such transmission lines often include bends that turn the direction of energy propagation (i.e., change the direction of field orientation) from one direction to another. A right angle transmission line bend, for example, turns the direction of energy propagation around 90 degrees. One drawback is that transmission line bends introduce losses.
One type of loss, called a return loss, relates to the energy that is reflected back from the transmission line bend. Return losses can be created due to capacitance and inductance being formed around the transmission line bends. For example, capacitance may arise through charge accumulation at the right angle transmission line bend, particularly, around the outer point of the transmission line bend where the electric fields concentrate. Inductance may arise due to current flow constriction. In addition, the change of field orientation at the right angle transmission line bend is influenced by mode conversions. These influences significantly increase the return loss.
Focusing on the return loss, several techniques have been implemented in the past to compensate for the transmission line bends in order to reduce the effect of the capacitance and inductance. For example, the transmission line bends may be mitered and rounded where the miter technique removes metal where there is no current flow, and that reduces the capacitance and inductance. Doing so improves the voltage standing wave ratio (VSWR) and reduces the return loss.
A coplanar waveguide (CPW) is an attractive choice for the development of monolithic microwave integrated circuits (MMICs). A CPW is formed from a conductor separated from a pair of ground planes, all on the same plane, atop a dielectric medium. Several advantages of CPWs include ease of shunt and series connections, low radiation, low dispersion, and avoidance of the need for thin fragile substrates. One drawback of a prior art CPW bend is that the two slots and the two ground planes on each side of the center conductor have different lengths. The different lengths cause unwanted slot-line and parallel plate modes which tend to radiate and reduce the overall performance of the transmission line.
FIG. 1A is a schematic view of a prior art CPW bend 104 that utilizes air-bridges 102 for performance improvements. FIG. 1B is a schematic view of a prior art chamfered CPW bend 106 that utilizes air-bridges 102 for performance improvements. The prior art CPW may include a center signal plane and a pair of ground planes. The center signal plane may have a width W, and it may be spaced between the pair of ground planes with a slot width S. Referring to FIGS. 1A and 1B, the placement of air-bridges 102 near the CPW bends 104 and 106 has been used to eliminate unwanted slot-line and parallel plate modes. However, the inclusion of air-bridges 102 may add unwanted capacitance on the transmission lines which can further degrade the CPW performance. CPW performance is especially important at mm-wave frequencies.
FIG. 2 is a schematic view of a prior art CPW bend that utilizes high-impedance transmission line sections 204 under the air-bridges 202 for performance improvements. The high-impedance transmission line sections 204 under the air-bridges 202 are narrower and therefore add less parasitic capacitance on the transmission line. However, the high-impedance transmission line sections 204 require the addition of short matching networks.
Although the foregoing techniques are helpful in reducing the return loss for the transmission line bends, additional improvements can be made to improve the VSWR and reduce the return loss. Moreover, they require the fabrication of air-bridges which is complex. Therefore, a need exists in the art for systems and methods for improving the performance of CPW bends at mm-wave frequencies without the need for air-bridges.